1. Field of the Invention
This invention relates to a flow control apparatus in which a part of a fluid discharged from a pump is returned to a suction side of the pump by sliding a flow regulating spool in a valve bore, thereby controlling the flow delivered to a destination, and more particularly to a flow control apparatus which reduces inversely the delivering flow in a range of a large quantity of the discharged fluid.
2. Description of the Related Art
In many fluid delivery system, the flow delivered to the destination must be controlled in accordance with requirements of the destination. In such a system, a pump which is a source of the delivered fluid is provided with a flow control apparatus which controls the delivering flow by returning a part of the fluid discharged from the pump to the suction side.
For example, in a hydraulic power steering apparatus in which a hydraulic fluid is delivered to a hydraulic actuator disposed in a steering mechanism and the steering assisting force is obtained by a force generated by the hydraulic actuator, a hydraulic pump which is a generating source of the hydraulic fluid is generally driven by an engine, and the discharging flow from the hydraulic pump is increased as an automobile speed increases. On the other hand, the road reaction force acting on the wheels during the steering operation is great when the automobile stops or runs at a low speed and small when the automobile runs at a high speed. Therefore, a power steering apparatus which is operated by a delivered hydraulic fluid is required to generate a steering assisting force which increases or decreases depending upon whether the automobile speed is low or high. Accordingly, it is required that a hydraulic pump can maintain its delivering flow to a power steering apparatus at a substantially constant level irrespective of the quantity of the discharged fluid and more preferably, in a range of a greater quantity of the discharged fluid in a high speed running of the automobile, reduce inversely the quantity of the fluid delivered to a power steering apparatus. A hydraulic pump is provided with a flow control apparatus for accomplishing such an automatic regulation o f the quantity of the delivered fluid.
In such a flow control apparatus, a supply chamber to which a fluid discharged from a hydraulic pump is supplied and a delivery chamber communicating with the destination are formed in a valve bore of the pump housing, and a throttle section is formed between these chambers. Furthermore, a flow regulating spool is disposed so that its sides respectively face the supply chamber and a pressure chamber in communicating with the delivery chamber. The flow regulating spool is operated by the pressure difference between these two chambers (i.e., by the pressure different across the throttle section). The operation of the flow regulating spool causes a part of a hydraulic fluid supplied to the supply chamber to return to the suction side of the hydraulic pump. In accordance with the operating position of the flow regulating spool, the fluid supplied to the supply chamber is distributed to the delivery chamber and a circulation passage communicating with the suction side. The pressure difference across the throttle section upon which the operating position of the flow regulating spool depends corresponds to the quantity of the fluid passing the throttle section (i.e., the quantity of the fluid delivered to the destination). Hence, the operation of the flow regulating spool causes the quantity of returned fluid to be increased in accordance with the increase of the quantity of the delivered fluid, thereby maintaining the quantity of the delivered fluid at a substantially constant level.
A flow control apparatus has been practically used in which the throttle section is composed of a fixed throttle through which the entirety of the fluid supplied to the supply chamber passes, and a variable throttle varying its area in accordance with the pressure difference across the fixed throttle. Since the flow path resistance of the variable throttle increases with the increase of the quantity of the supplied fluid, this flow control apparatus can decrease the quantity of the delivered fluid inversely as the quantity of the supplied fluid (i.e., the fluid discharging flow of the pump) increases, and hence is widely used as one satisfying the above-mentioned requirements of a power steering apparatus.
A typical example of a flow control apparatus of this kind is disclosed in U.S. Pat. No. 4,361,166. FIG. 1 is an enlarged sectional view illustrating the main portion of this flow control apparatus.
As shown in FIG. 1, this flow control apparatus comprises a discharge passage 10 which is in communication with the discharge side of a hydraulic pump and a circulation passage 11 in communication with the suction side thereof. These passages 10 and 11 are formed in a housing of a hydraulic pump and open while being separated along the axial directon by an adequate distance in a valve bore 1 which is communicated with the destination of a hydraulic pressure through a delivering union 3 threadably fixed to an open end thereof. At the innermost position of the valve bore 1, a flow regulating spool 2 is inwardly fitted so as to be slidable in the axial direction. The flow regulating spool 2 is urged toward the open end (the left side of the figure) by a compressed spring (not shown) interposed between the spool 2 and the bottom face of the valve bore 1, to be pressed against the forward end of the delivering union 3 which is extended so as to close the open end of the discharge passage 10.
An extended portion 30 of the delivering union 3 has a cylindrical internal cavity which is divided by a throttle plate 31 fitted into the cavity into a supply chamber 5 and a delivery chamber 6 which communicates with the destination. The supply chamber 5 is in flow communication with the discharge passage 10 through a fixed throttle 32 which is configured as a hole penetrating the periphery wall of the extended portion 30. The supply and delivery chambers 5 and 6 are placed in communications with each other by a throttle hole 31a penetrating the center portion of the throttle plate 31 and also by a plurality of throttle holes 31b which are arranged with a uniform space around the hole 31a.
The internal pressure of the delivery chamber 6 is led to the back side of the flow regulating spool 2 through a communicating passage 12 which is parallel with the valve bore 1. The flow regulating spool 2 is caused to slide toward the innermost portion of the valve bore 1 against the resilience of the compressed spring by the pressure difference between the supply and delivery chambers 5 and 6 which is generated by the passing of the fluid through the throttle holes 31a and 31b, thereby increasing the opening area of the circulation passage 11 which opens in the valve bore 1. This causes a part of the fluid supplied into the supply chamber 5 to return to the suction side through the circulation passage 11, with the result that the quantity of the delivered fluid outputted via the delivery chamber 6 is decreased.
In the supply chamber 5, a throttle spool 33 is fitted so as to be coaxially slidable. A coil spring 34 which urges the throttle spool 33 and the throttle plate 31 in opposing directions is interposed between the throttle spool 33 and the throttle plate 31. The throttle spool 33 comprises a fluid passage bore 33a which opens at the axial portion in the side of the flow regulating spool 2 and which is branched into a pair of bores slanting radially and outwardly so as to open in the side of the throttle plate 31. The sliding movement of the throttle spool 33 in the urging direction of the coil spring 34 is restrained by a stopper 35 engaged into the inner wall of the extended portion 30 in the side of the flow regulating spool 2. Between the stopper 35 and the throttle spool 33 is, formed an annular chamber which is in communication with the discharge passage 10 through a pressure lead bore 36 which penetrates the periphery wall of the extended portion 30.
The fluid supplied from the discharge passage 10 into the supply chamber 5 through the fixed throttle 32 advances to the front side of the throttle plate 31 via the fluid passage bore 33a formed in the throttle spool 33, and is then introduced into the delivery chamber 6 through the throttle holes 31a and 31b which penetrate the throttle plate 31, and delivered to the predetermined destination. At this time, the throttle spool 33 is moved to slide against the resilience of the coil spring 34 toward the throttle plate 31, by the difference between the internal pressure of the supply chamber 5 and that of the discharge passage 10 which is led via the pressure lead bore 36 into the annular chamber formed between the throttle spool 33 and the stopper 35 (i.e., by the pressure difference generated across the fixed throttle 32), so that the throttle hole 31a at the center of the throttle plate 31 is closed by a projection 33b formed at the front end of the throttle spool 33. Namely, the throttle holes 31a and 31b formed in the throttle plate 31 function as a variable throttle which decreases its throttle area in accordance with the increase of the pressure difference generated across the fixed throttle 32 by the supply of the hydraulic fluid into the supply chamber 5. In accordance with the pressure difference generated across the variable throttle by the supply of the introduced fluid into the delivery chamber 6, the flow regulating spool 2 slides as described above, thereby adjusting the quantity of the fluid introduced into the delivery chamber 6, i.e., the quantity of the fluid delivered to the destination.
Therefore, in a hydraulic pump provided with the flow control apparatus, the quantity of the delivered fluid increases proportionally as the rotational speed of the pump increases over a relatively small range. After the flow regulating spool 2 has been caused to begin the sliding movement by the increase of the quantity of the delivered fluid, however, the quantity of the fluid returned to the circulation passage 11 increases in accordance with the increase of the quantity of the fluid supplied form the discharge passage 10, with the result that the quantity of the fluid delivered to the destination is maintained at a substantially constant level irrespective of the increase of the pump rotational speed. When the quantity of the supplied fluid increases further, the throttle spool 33 is caused to begin to slide by the pressure difference generated across the fixed throttle 32. During the period from this time to a time when the throttle hole 31a at the center of the throttle plate 31 is closed by the projection 33b formed at the front end of the throttle spool 33, the throttle area of the variable throttle which consists of the throttle holes 31a and 31b decreases, resulting int hat its flow path resistance increases. This causes the increasing rate of the quantity of the returned fluid which is produced by the sliding movement of the flow regulating spool 2, to exceed the increasing rate of the quantity of the supplied fluid, and the quantity of the fluid delivered to the destination is decreased inversely as the rotational speed of the pump increases, whereby the quantity of the delivered fluid varies int he manner shown in FIG. 2. This manner of varying the quantity of the delivered fluid is desirable in a generating source of a hydraulic fluid for a power steering apparatus.
However, a conventional flow control apparatus having such a configuration has a drawback that, since the entire quantity of the fluid introduced into the delivery chamber 6 passes through the fluid passage bore 33a formed in the throttle spool 33, a large dynamic pressure acts on the throttle spool 33, and particularly, in a range of a greater quantity of the fluid introduced into the delivery chamber 6, the operation of the throttle spool 33 is unstable, and therefore it is difficult to stably obtain the range of the reduced quantity of the delivered fluid which is shown n FIG. 2. This drawback maybe overcome by enlarging the area of the fluid passage bore 33a to reduce the velocity of flow in the fluid passage bore 33a. However, the increase of the area of the fluid passage bore 33a formed in the throttle spool 33 which is coaxially fitted in the extended portion 30 of the delivering union 3 has a limitation. In order to eliminate the unstable operation of the throttle spool 33 which is caused by the dynamic pressure, it is required to make the bore of the throttle spool 33 large, causing a problem in that the overall size of the flow control apparatus becomes large.
Furthermore, such a conventional flow control apparatus has a complex shape in which the flow path from the supply chamber 5 to the delivery chamber 6 is widened outwardly at the branching portion of the fluid passage bore 33a and thereafter contracted toward the throttle hole 31a at the center of the throttle plate 31. In such a flow control apparatus, when a hydraulic pump is started in a cold district, for example, the flow of a high viscous fluid is impeded, with the result that a very high surge pressure is generated. This may cause the hydraulic pump at the upper stream and the piping system at the lower stream from the delivering union 3 to the destination to be damaged. Moreover, such a flow control apparatus suffers from the defect that the high surge pressure generates a harsh noise (gargle sound) which prolongs for a long period of time.